The synthesis of diamond by various methods is well known and well established. One such example is the synthesis of diamond under high pressure and high temperature (HPHT). There are two principle methods employed and both are from solution. The one method is a temperature gradient method and the other is an allotropic change method. In the temperature gradient method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of source material and the growing crystal as a result of a temperature difference between the two. The carbon that is present in the higher temperature region migrates to the seed crystal, which is positioned in the lower temperature region, via a solvent/catalyst material that separates the source material and seed crystal. Such a temperature gradient method is described generally in U.S. Pat. No. 4,034,066.
Almost all diamond synthesised from solution contains nitrogen and is diamond of type Ib.
When growing diamonds of the type IIa, which have a nitrogen content generally lower than 5 ppm (parts per million by mass), removal of nitrogen from the starting materials is necessary. This is typically achieved by using a nitrogen getter. The nitrogen getter or agent is added to the solvent/catalyst, which is typically a molten alloy of the transition metals cobalt, iron and nickel. This agent has the effect of preferentially sequestering the nitrogen in the metallic melt, either as a solute or as a precipitated nitride or carbo-nitride. Such agents are typically elements like titanium, zirconium and aluminium.
Annealing is a method used in the field of materials science to improve the crystal domain size and perfection of the domains generally and is well known. Usually an elevated temperature is used but with no increased pressure. For example, U.S. Pat. No. 5,908,503 discusses the use of a high temperature furnacing stage, typically using temperatures of 1100 to 1600° C. and low pressures, using a non-oxidising atmosphere to improve the crystalline perfection of diamond. The non-oxidising atmosphere is a requirement to prevent the oxidation and loss of the diamond crystal during treatment. The process of U.S. Pat. No. 5,908,505 uses low pressures and the inventors thereof actively discourage the use of high pressures in the annealing process, as it is stated that defects are incorporated into a diamond crystal during the course of lowering the pressure and temperature back to normal temperature and pressure levels.
Annealing of extended lattice defects in diamond requires diffusion of carbon atoms. The diamond lattice is a very tightly bonded lattice and diffusion is restricted except under certain conditions. Increasing temperature increases diffusion, but increasing pressure generally reduces it. In type Ib diamond the presence of significant levels of nitrogen in the diamond significantly enhances diffusion. Although some prior art (V. D. Antsygin's article in Optoelectronics Instrumentation and Data Processsing (1998) No. 1 p 9) have shown an improvement on the crystalline perfection in BARS grown type Ib diamond on annealing at 2100° C., the extended defects and annealing mechanism in type Ib and IIa are quite different and the prior art teaches that it is very difficult to remove structural defects in diamond containing a low nitrogen concentration.
A need exists for a diamond material having a low density of extended defects (crystalline perfection) in combination with a low and controlled impurity levels and in particular low levels of nitrogen such as found in type IIa diamond. A further need exists for preparing such diamond, where crystalline perfection relates to minimizing the concentration of extended defects such as dislocations and stacking faults.